As the inks for use in ink-jet printing, conventionally, inks the main components of which are a water-soluble dye and a liquid medium were the leading one. However recorded materials thereby do not have, for instance, sufficient water resistance, light resistance, and ozone resistance, due to the characteristics of the water-soluble dye used therein, so that there has been developed an ink in which a pigment is dispersed instead of the water-soluble dye in the water-soluble medium.
However, in such a pigment ink, a dispersant and a dispersion resin are generally often used in order to disperse the pigment in the ink. A most widely used polymeric dispersant inherently has a problem concerning the lowering of the storage stability of the ink that when the amount added is excessive, printing becomes impossible in a recording method such as ink-jet printing due to the elevation of the viscosity of the ink, while when the amount added is insufficient, the dispersibility of the pigment is lowered so that the coagulation and precipitation of the pigment takes place. Therefore, an improvement on an ink-jet recording liquid is desired that can facilitate the handling thereof.
For such improvement, there are known the following pigments and dyes for use in a recording liquid in which a pigment and a dye are used in combination: (1) a pigment and a dye which are chemically adsorbed by allowing the dye and a functional group on the surface of the pigment to react (refer to Patent Document 1), or (2) a pigment and a dye which are physically adsorbed (refer to Patent Documents 2, 3, 4, 5 and 6).
Of the above, in the former (1), the kind of the dye to be adsorbed is limited in accordance with the kind of the functional group of the pigment to be used, so that the combination of the dye and the pigment is limited and therefore, many limitations are to be confronted with in practical use thereof.
For example, from the viewpoint of the chemical reaction therebetween, when the pigment has a basic functional group on the surface thereof, it is necessary to combine a dye having an acidic group with the pigment, while when the pigment has an acidic functional group on the surface thereof, it is necessary to combine a dye having a basic group with the pigment. However, an aqueous recording liquid for ink-jet printing is usually a basic medium, so that a dye having an acidic group is selected when the dispersibility thereof in the medium is taken into account. Therefore, there occurs a problem that pigments having generally acidic functional groups such as a carboxylic group and a sulfonic acid group, cannot be used. Furthermore, in this method, when the chemical adsorption has proceeded 100%, the effect of dispersion stability is exhibited only by the functional groups which remain in the dye. However, in order to obtain practically usable dispersion stability only by the functional groups remaining in the dye, the density of the functional groups present on the surface of the pigment on which the dye is to be adsorbed is required to be above a certain level. However, in this case, it may occur that such adsorption does not proceed 100%, for instance, due to a steric hindrance between the dyes themselves, resulting in that the functional groups derived from the pigment and the functional group derived from the dye, which are different in polarity, coexist on the surface of the dispersion particles. Such a state may cause dispersion instability of each particle and particle association and cannot be said to be a most appropriate one for the dispersoid for use in an ink-jet ink, which is required to have storage stability for a long period of time.
In the latter (2), for the improvement on the water resistance and light resistance of a dye, and also on the chroma of a pigment because of the poor water resistance and light resistance thereof, and the insufficient chroma thereof, there are known an aqueous recording liquid comprising a pigment, a water-soluble dye and a high polymer dispersant with a molecular weight of from 1000 to 100000 used in combination (refer to, for example, Patent Document 2), a recording liquid comprising a dye having a sulfonic acid group or a sulfonate group, a liquid medium, a polymeric material which is soluble in the liquid medium, and a pigment (refer to, for example, Patent Document 3), and an ink-jet recording liquid containing a dye, a pigment, a water-soluble surfactant and water (refer to, for example, Patent Document 4).
However, the dispersions containing a pigment and a dye, disclosed in these prior art references, are prepared by mixing a pigment and a dye together with a polymeric dispersant or a polymeric material, or by mixing a dye with a dispersion of a pigment which has been dispersed in advance by use of a surfactant. In such preparation methods, there are merely obtained a dispersing effect which is attained by the dispersant such as a polymeric dispersant and a surfactant being adsorbed onto a pigment, and a color tone enhancement effect which is attained by the coexistence of a water-soluble dye. Therefore, these references neither recognize nor suggest the state of the absorption of the dye on the pigment, the control thereof, and the technical effects obtained thereby.
Furthermore, in the case of a pigment dispersion prepared by simply mixing a pigment, a dye and a polymeric dispersant or a polymeric material, a composite material composed of the pigment and the polymeric dispersant or the polymeric material, and a composite material composed of the dye and the polymeric dispersant or the polymeric material are simultaneously formed, thereby producing heterogeneity in the system. There is concern that such heterogeneity will cause problems in the physical properties in solution of a dispersion and a recording liquid, in particular, in the reproducibility and changes in the viscosity characteristics thereof with time.
In addition, as the polymeric dispersant disclosed in the above-mentioned Patent Document 2, there is used a dispersant with a weight average molecular weight of as high as 20000, so that the problems as caused in the system when the above-mentioned polymeric dispersant is used cannot be solved.
Furthermore, of the latter (2), there is proposed a recording liquid in which a dye is used as a dispersant for a pigment, instead of a surfactant or a polymer which is conventionally used for dispersing a pigment (refer to Patent Documents 5 and 6). However, when a recording liquid is prepared by use of a pigment dispersion in which only a dye is used as a dispersant, this preparation method has large limitations on an organic solvent and an additive to be used in the recording liquid since the dispersion stability of the pigment is largely lowered, depending upon an organic solvent and an additive used, so that there is a problem in practical use.
In particular, in the method disclosed in Patent Document 6, only a dye which has the same basic skeleton as that of a pigment is used, and a step of removing an unabsorbed dye from a dispersoid of the pigment and the dye by ultrafiltration or centrifugation is indispensable, so that from the viewpoints of the freedom of the selection of color tone, complexities, cost and others thereof, the method has problems in practical use.
Patent Document 1JP-A-9-151344Patent Document 2JP-B-60-45667Patent Document 3JP-A-5-247391Patent Document 4JP-A-8-218019Patent Document 5W/O 99/61534Patent Document 6JP-A-2000-273383